Langmuir 1992,0, 2284-2289
2284
Adsorption of Ammonia on Mica Surfaces Krishna G. Bhattacharyya Department of Chemistry, Gauhati University, Guwahati- 781014, Assam, India Received March 9,1992. In Final Form: June 19,1992 Adsorption of ammonia on muscovite mica surfaces a t room temperature was studied in an ultrahigh vacuum glass adsorption-desorption apparatus. Ammonia was found to adsorb with first-order desorption kinetics. Two desorption peaks were obtained around 318 and 450 K, the activation energies of desorption of the two states being 76 and 122 kJ mol-'. A cleaned, air-cleaved mica surface showed almost equal activity toward ammonia as did a vacuum-cleavedsurface. ,Thelow temperature statehad a saturation coverage about 10 times as large as the high temperature state. The adsorption of ammonia is explained by proposing the presence of acidic sites on the mica surface. X-ray photoelectron spectroscopy measurements also revealed a small broad N 1s peak around 400 eV after exposure of the mica surface to 600 langmuirs of ammonia a t room temperature.
Introduction Natural muscovite mica of the ideal formula KAl2Si3AlOlo(OH)2 can be obtained in high grade single crystal form. The layered structure of this hydrous aluminosilicate has been very well The basic unit consists of three layers; an octahedrally coordinated layer of metal atoms, mostly aluminum, is sandwiched between two identical (Si,A1)203 tetrahedral layers with inward pointing vertices. On the average, one-fourth of the tetrahedral atoms are aluminum, and thus the 2:l sheet has a net negative charge which is balanced by a layer of potassium ions. The edge-on view of the ideal muscovite structure is shown in Figure 1. The mica surface is regarded as very inert due to the presence of the potassium ions on the surface. Unlike the common solid acid catalysts, the Si/A1ratio in mica is very small (nearly 1.0) and generation of acidic sites on the mica surface has been considered almost impossible. Liiffler e t observed the depletion of both potassium and aluminum atoms from the mica surface on prolonged heating. How and in what form the potassium ions leave the mica surface has remained uncertain. The synthetic mica montmorillonites, having remarkable similarity in structure to the muscovite mica (excepting that the interlayer cations are ammonium ions), have been shown to produce an acidic surface on heating.6 The present work was aimed at a search for acidic sites, if any, on the mica surface through temperature programmed desorption (TPD) of ammonia.
Experimental Section A glass ultrahigh vacuum (UHV) apparatus equipped with a Bayard-Alpert type ion gauge and a VG Anavac-2 quadrupole mass spectrometer was used for adsorption-desorption experiments. The apparatus with its sample holder assembly, in situ cleaving and heating arrangements,and other details have been described elsewhere? The apparatus had a very low effective (1) Radoslovich, E. W. Acta Crystallogr. 1960, 13,919.
(2) Bragg, L.;Claringbull, G. F.; Taylor, W. H. In Crystal Structure of Minerals;Come11 University Press: Ithaca, NY, 1965; Chapter 13, p 253. (3) Baily, S. W.In Crystal Structures of CZay Minerals and Their X-ray Identification; Brindly, G. W., Brown, G., Eds.; Mineralogical Society London, 19W, Chapter 1. (4) Deer, W. A.; Howie, R. A,; Zussman,J. In Rock-forming Minerals; Longman: London, 1962; Vol. 3 (Sheet Silicates). (5) Gffler, D.; Hailer, G. L.; Fenn, J. B. J . Catal. 1979,57,96. ( 6 ) Wright, A. L.; Granquist,W. T.; Kennedy, J. V. J . Catal. 1972,25, 65. (7) Bhattacharyya, K. G. Langmuir 1989,5,1155.
0 0 0 0
K+ 0 SI, A I
2.258i
O.OH
1
AI
2.120i
1:
0.OH
2.258
S I , AI
0
K
I
s
I
/
I
1
I
1
Figure 1. Edge-on view of the ideal muscovite mica structure. volume of 2.98 L and was thus extremely sensitive to small pressure fluctuations. Natural muscovite mica, grade 5, of highest purity and supplied by Startin and Co., London, was used in all the experiments. The sample size was 28 mm X 23 mm and was less than 0.25 mm thick. Both faces of the sample could be cleaved in UHV by an elaborate mechanism operated magnetically from outside. The mica samples were heated radiatively with a tungsten filament, placed at a distance of about 15 mm and the temperature was controlled with a specially designed linear temperature programmer unit.7 A current of 4 A at 14V was necessary to obtain a temperature of 600 K on the mica surface. Temperature was measured with the help of a chromel-alumel thermocouplewhich was pushed carefully inside the mica layers. The tungsten filament was also used to atomize hydrogen for cleaning the aircleaved surface with hydrogen atom bombardment. The samples were degassed in UHV at 570 K before exposure toammonia. Duringthis process, a large amountof gas consisting of water vapor, nitrogen, carbon monoxide, carbon dioxide, methane, hydrogen, oxygen, and some hydrocarbon fragments were released. Repeated degassing could not completely eliminatewater vapor. It is possible that occluded water vapor evolves with slow diffusion to the surface while new water molecules are generated through a slow dehydroxylation process as the temperature goes up. Ammonia for the adsorption experimentswas generated from ammonia solution (BDHAnalaR grade,35% ammonia)by double freeze-pumpthaw process with the aid of two liquid nitrogen traps in series. All exposure experiments were done at room temperature,and after each experiment,the system was pumped
0743-7463/92/2408-2284$03.00/0 0 1992 American Chemical Society
Adsorption of Ammonia on Mica Surfaces
Langmuir, Vol. 8, No.9,1992 2285
Q M S SIGNAL AR 81 T RARY UNIT o a o b O C
TIME/SEC TEMP.
/K
tl 0
10
I
1
298
20
30
1
1
392
40 I
! I
48 6
Figure 2. A typical desorptionspectrum for room temperature adsorption of ammonia on the air-cleaved, hydrogen atom bombarded mica surface. The time was recorded from the moment when the mica temperature began to rise. down to the base pressure (2 X 1O-gmbar) to get rid of any weakly held species of ammonia. X-ray photoelectronspectra for ammonia adsorption on mica were taken with a VG ESCALAB MK I1 system.8 A mica sample of 1 cm2 area was fixed on a nickel stub holder with tantalum clips for these experiments. The top face could only be cleaved with a special arrangement. The X-ray photoelectron spectroscopy (XPS)experiments revealed a large carbonaceous overlayer on the air-cleaved mica surface. There was however no facilityfor cleaningthe surface with hydrogenatom bombardment in the ESCALAB.
Results and Discussion The air-cleavedmica surface without any pretreatment showed no detectable affinity toward ammonia. The aircleaved, hydrogen atom bombarded surface and the vacuum-cleaved surface, both after prolonged degassing, adsorbed ammonia. A typical TPD pattern, obtained by monitoring mass 17 partial pressure, is shown in Figure 2. Both mica surfaces produced identical patterns. The first two peaks appeared even before the mica surface recorded any rise of temperature. In a blank experiment, without the mica sample in the system, these two peaks were the only ones obtained, and hence these peaks were assigned to desorption from the tungsten filament. By monitoring masses 16 and 18along with mass 17, the first peak was found to be due to water vapor and the second peak due to ammonia. This is in agreement with the reportg of only a single desorption peak for ammonia on (100) oriented polycrystalline tungsten. Peaks C and D were obtained at temperature ranges of 300-350 K and 375-510 K, respectively. Absence of these peaks on blank runs established them as peaks due to desorption from the mica surface. This was confirmed by a series of experiments keeping the sample at a right angle to the filament during TPD runs (at this position, the temperature of the mica sample rose very slowly while the filament became white hot almost instantaneously); the two peaks then appeared after a prolonged interval. This (8) Bhattacharyya, K. G. Ph.D. Thesis, University of London, 1984. (9) Reed, A. P. C.; Lambert, R. M. J. Phys. Chem. 1984,88,1964.
1
1
3
330
370
410
450
490
530
TEMP./K
Figure 3. Shift in the TPD peak maximum with heating rate for room temperature adsorption of ammonia on the air cleaved, hydrogen atom bombarded mica surface: exposure, 3.7 X 1016 molecules cm-2;heating rates, (a) 3.8, (b) 5.6, (c) 7.5 K s-l. Table I. Peak Maximum Temperatures at Different Heating Rates for TPD of Ammonia from Mica Surfacer peak maximum temperature, TJK heating low-temperature peak high-temperature peak rate, air-cleaved vacuum-cleaved air-cleaved vacuum-cleaved BIKs-~ mica mica mica mica 2.2 310.5 310.0 460.6 461.0 3.0 313.0 312.0 463.0 463.0 3.8 315.0 316.0 466.0 465.6 4.7 317.5 316.8 457.8 468.0 6.6 319.0 318.6 460.5 461.0 6.0 321.0 320.8 463.0 462.6 7.6 323.0 322.6 466.6 464.0
also showed that the peaks were not due to desorption from the glass walls. Peak D had a shoulder on the low temperature side, but it could not be resolved and the peak was considered as a single peak for all purposes. The low temperature side of peak C remained enclosed within the f i i e n t peaks. Both peaks C and D were found to follow first-order desorption kinetics from their coverage-independentpeak maxima at constant heating rate as well as from their asymmetry about the peak maxima. The peaks shifted to higher temperatures with increasingheating rate. This is shown in Figure 3 for three different heating rates. The shift in peak maxima with heating rate for the air-cleaved, cleaned surface and the vacuum-cleaved surface is given in Table I. Both surfaces show good correspondence. Redhead's formulationlowas used to evaluate the desorption kinetics. The desorption rate for unit surface area is given by
-
-1
= Nory,, exp(- E dt RT where n is the order of desorption, u is the surfacecoverage (10)Redhead, P.A. Vacuum 1962,12,203.
2286 Langmuir, Vol. 8, No.9, 1992
Bhattacharyya 112
: b )
215
211
219
( l/Tp)/ K / ~ o - ~ Figure 4. Plots of In (T,2//3)versus (UTp)for (a) the low-temperature peak and (b) the high-temperaturepeak. Closed circlea represent air-cleaved and open circles vacuum-cleaved surfaces of mica. Table 11. A Few Prominent Features of Ammonia Adsorption on Mica Surfaces at Roam Temperatare low-temperature state high-temperature state air-cleaved mica vacuum-cleaved mica air-cleaved mica vacuum-cleavedmica features activation energy, E/kJmol-' 76.7 75.2 121.3 122.8 pre-exponential, u 1 / ~ - l 1.2 x 10'2 1.8 X 10l9 1.8 X 1018 8.6 X 10" saturation coverage/moleculea 2.4 X 1Ola 1.8 x 1013 3.6 X 1O12 3.9 x 10'2 initialsticking probability 2.8 X 1W9 7.8 X 1od 3.9 x lo-' 6.5 X lod
in molecules cm-2, No is the number of adsorption sites cm-', 8 is the fractional surface coverage, d N 0 , vn is the pre-exponential factor, and E is the activation energy of desorption. In this work, the temperature of the mica surface followed a linear relationship with time, i.e. T = To+ Bt, where B is the heating rate in K 8-l. Assuming negligible readsorption and also constancy of the activation energy with surface coverage, the eq 1can be solved for a firstorder desorption process to obtain
at the peak maximum temperature, Tp' v1 is the fiistorder pre-exponential term. Plots of In (Tpz//3) versus (UTp)for the two ammonia desorption peaks on mica surfaces are shown in Figure 4. The linear plots further confiim first-order desorption kinetics. The activation energies and the pre-exponential factors obtained from these plots are given in Table 11. The saturation coverages for ammonia on mica and the initial sticking probability values obtained from the plots of coverage versus exposures are also given. It is seen that the air-cleaved, cleaned mica surface had both a higher saturation coverage and a higher stickingprobability than those of the vacuum-cleaved surface for ammonia adsorption in the low temperature state. The high temperature state of adsorption had a lower sticking probability for the vacuum-cleaved surface, but the surface coverages on both types of surfaces were similar. Ammonia is a relatively strong base having almost equal preference for Bronsted and Lewis acid sites. First-order
desorption rules out dissociative adsorption forming species like -NHz and NH on the surface, because it will be unlikely to have these species on adjacent sites during desorption. The low temperature state is obviously a very weakly held species of ammonia on apparently weak acidic sitas. Strongly acidic sites must be invohed in the high temperature state. From Table 11, it can be men that the high-temperature state has a saturation coverageof about one-tenth of that for the low-temperature state, thue indicating that the number of weak acid sites is far in excess of the number of strong acid sitm on the mica surface. The presence of acid sites on mica surfaces ie likely to follow a scheme similar to that for the zeoliteell
Brksted acid (I)
0
HO
o/ \ o d
\0
Lewis acid (11)
The trivalent aluminum in (11) behaves as a Lewis acid site. (11) Uytterhoeven, J. B.;Chrktner,L.G.;Hall, W.K.J. Phys. Chem.
1966,69,2117.
Langmuir, Vol. 8, No. 9,1992 2287
Adsorption of Ammonia on Mica Surfaces
Several authors have reported depletion of potassium ions from the mica surface on prolonged heating.5 The mechanism of potassium removal may be as follows
-
K+(H20) KOH + H+ Thermodynamic considerations for such processes have not been worked out, but similar schemes have been propo~ed’~J~ for presence of acid sites on alkali metaland alkaline earth metalzeolites. Each ox gen hexagon14in the surface of mica has an area of 24 and therefore the aluminum atom should be accessible to small molecules like ammonia through the hexagonal holes created by the removal of potassium and subsequent dehydroxylation to 11. Ammonia adsorption on mica is therefore likely to follow two paths: (a) Electron pair donation to the trivalent aluminum of the Lewis acid (11)
x2
I1
On desorption, I11 reverts back to II. (b) Formation of an ammonium form after ammonia adsorption on I
I
I \’
Ammonia desorptionoccursthrough the releaseof a proton which may either be hydrated to form a hydroxonium ion or attack a bridge oxygen forming structure 11. Both III and IV represent only a single state of adsorbed ammonia on mica surface since one form can be transformed into the other by migration of a proton. In a scheme similar to the zeolites,dehydroxylation of 11can also produce another kind of Lewis acid site in mica
V
Ammonia can add to the electron deficient Si+ of V very strongly
VI
Zeolites are known to dehydroxylate at a temperature of 800 K or more. Mica samples were not heated above 600 (12)Angell, C.L.;Schaffer, P.C . J. Phys. Chem. 1966,69,3463. (13)Ward, J.W.In Zeolite Chemistry and Catalysis; Rabo,J. A, Ed., ACSMonograph 171;AmericauChemicalsoCiety:Weehghn,DC, 1976; p 118. (14)Barrer, R. M. In Zeolites and Clay Minerals as Sorbents and Mokcular Sieves; Academic Press: New York, 1978; p 407.
K, but at this temperature, some dehydroxylation could not be ruled out. The hydrogen atom bombardment was ale0 likely to remove some of the bridge oxygen atoms leaving behind trivalent aluminum and trivalent silicon. Evolution of water vapor during hydrogen atom bombardment might be an indication of dehydroxylation. Ammonia adsorbed in the form of either 111or IV is likely to be loosely held, whereas in VI, ammonia is likely to be held very strongly. On this basis, the low temperature state of ammonia on mica may be attributed to adsorption on sites of type I or 11and the high-temperature state of adsorption on sites of type V. Much larger uptake of ammonia in the low-temperature state as compared to that in the high-temperature state supports thishypothesis,because the mica surface can be expected to have very few dehydroxylated sites (typeV) while the type I or I1 sites may be quite numerous. The number of the later type of sites is obviously equal to the number of potassium ions removed from the surface. Assuming that a potassium ion sits on an oxygen hexagon of area 24 A2, 1cm2 of the mica surface holds about 4.2 X 1014potassium ions, which are equally divided on cleaving between the two cleaved faces. A freshly cleavedsurfacelShas therefore about 2.1 x 1014potassium ions cm-2. That the saturation coverage for the low temperature state of ammonia is of the order of 1013molecules cm-2 is an indication that only one-tenth of the surface potassium ions make way for adsorption of ammonia. Significantly it was observed that a freshly vacuum cleaved, untreated mica surface showed some adsorption only at the low-temperaturestate. This is consistent with the above proposition because while the vacuum cleaved surface is likely to have a few type I or I1 si-, no site of type V can be expected beforethe mica surfaceis subjected to heating. Ammonia being a small molecule can enter into the interior of mica and may be held by a dipoledipole type of interaction or by hydrogen bonding. On heating, desorption of ammonia will take place through diffusion from the interior. It was however not possible in this work to investigate further into this type of interaction. Kinetic data on TPD of ammonia for materials comparable to mica are scarce. Topsoe et al.lSshowed three chemisorbed states of ammonia on ZSM-5 zeolitesat 333373,423-473,and 693-773 K, all Of which followed firstorder desorption kinetics with activation energies of 84.8, 97.0, and 163.0 kJ mol-l, respectively. No definite assignment was made, but various possibilities including interaction of ammonia with surface oxygen or hydroxyl groups by hydrogen bonding were suggested for the fmt two states. The third state (693-773K) was asaigned to adsorption at strong BrBnsted and/or Lewis acid sites. Hidalgo et al.17 observed two states of chemisorbed ammonia, both following first-order kinetica, on mordenite, ZSM-5, Y-faujasite, and other cation-exchanged and dealllminated zeolitq the adsorption was at 373 K. The TPD peak maxima were around 420 and 680 K with activation energies of 67 and 145 kJ mol-l. X,Y,and ZSM-5zeolites were found to desorb ammonia in seven stspslS between 303 and 937K. The sorption of ammonia in a series of La3+-,Ca2+-,and H-exchanged Y-zeolites was also reported193’ in the temperature range 333-483 ~~
(16)Parker, J. L. J. Colloid Interface Sci. 1990,134, 449. (16)Topsoe, N.; Pedersen, K.; Derouane, E. G. J. Catol. 1981,70,41. (17)Hidalgo,C.V.; Itoh, H.; Ha&, T.;Niwa, M.; Muralrami, T.J. Catal. 19&(,86,362. (18)Choudhury, V. R.;Pataakar, S . G. Zeolites 1986,6,307. (19)Shiralkar, V. P.;Kulkami, S . B.J . Colloid Interface Sci. 1985, 108,l.
2288 Langmuir, Vol. 8, No. 9, 1992
K. Modified ZSM-5 zeolites showed three TPD peaks for ammonia at 548,598,and 773 K,respectively, which were assigned to interaction of ammonia with framework aluminum cations, sodium cations, and BrGnsted acid sites.z1 Earlier, Parker et aLZzfound two states of adsorbed ammonia, a weakly bound physisorbed state and a strongly bound chemisorbed state on HZSM-5 zeolites. A detailed investigationz3of ammonia adsorption on six different zeolites revealed three desorption peaks in the temperature regions of lees than 473 K, 473-673 K,and higher than 673 K. The first peak was attributed to physically adsorbed or weakly chemisorbed ammonia molecules, the second peak to ammonia molecules adsorbed on zeolite hydroxyl groups, and the third peak to strong BrGnsted and/or Lewis acid sites generated after dehydroxylation. The activation energy of desorption for the high-temperature stateof ammonia on mica (122kJ mol-') indicates that the adsorption sites are quite strongly acidic. The desorption maxima at 450-465 K for this state show some similarity to the second state of Topsoe et al.,'s the first state of Lok et al.,23 and the low-temperature state of Hidalgo et al.17 for ammonia on zeolites. Limitations in heating arrangement prevented this author from looking into any other high-temperaturestate of ammoniaon mica. It must be pointed out that the TPD technique alone cannot be used to relate strong and weak acid sites to discrete positions on the surface. However, the present work clearly eetablishes the presence of two kinds of acidic sites on mica surfaces. Adsorption of hydrogen, oxygen, carbon monoxide, carbon dioxide, water vapor, methanol, pyridine, and 1butene was also tried at room temperature on both aircleaved and vacuum-cleaved mica surfaces. TPD measurements were done up to a temperature of 600 K. No chemisorption of hydrogen and oxygen was detected in agreement with the earlier observationsof Poppa and Elli0tZ4and of Dowsett at el?6 Chemisorption of carbon monoxidewas also not observed. The adsorptionof carbon dioxide has already been reported.' TPD after exposure to water vapor was uninformativeas there was a continuous evolution of water vapor also from the glass walls, which completely masked chemisorbed states, if any, from mica. Methanol also did not show any detectable chemisorption indicating the obsence of strong basic sites on the mica surface. Unlike ammonia, no adsorption of pyridine or 1-butene was detected. These molecules being quite large, their adsorption is likely to be dependent on steric factors, and thus the results were not unexpected. In XPS measurements,ammoniaadsorptionwas noticed on the air-cleaved mica surface after degassing for more than 18h a t 500 K in UHV. THe wide scan photoelectron spectrum after exposure to 600 langmuira of ammonia at room temperature isshown in Figure 5. Before taking the spectrum, the chamber was pumped down to near 2 X 10-lombar, which was likely to remove any weakly bound species of ammonia on mica. It is thus possible that the f i t of the two states of the TPD experimentswould escape detection. The N l a peak was very small and broad with a binding energy in the range of 399.7-400.0 eV after being (20) Shiralkar, V. P.; Kulkami, S. B. J. Colloid Interface Sci. 1986, 109. 116. (21) Reechetilowski, B. U.; Wendlandt, K. P. J. Chem. Soc., Faraday Tram. 1989.85.2941. (22) P&ker,L. M.; Bibby,D. M.; Meinhold,R. H.Zeolites 1986,6,3&4. (23) Lok, B. M.; Marcus, B. K.; Angell, C. L. Zeolites 1986,6, 186. (24) Poppa, H.;Elliot, A. G. Surf. Sei. 1971,24, 149. (25) Dowaett, M. G.; King, R. M.;Parker, E.H. C.J. Vac. Sci. Technol. 1977, 14, 711.
I " ' - ~ " " I ' " ' " ' '
1
-
l " " ~ " " l ' "
" I
'
A 1 XPS
Max Count Rate 10170
4
-
I v1
c 0 4J I
0
.r
L..
, , I , .,
,
I , .,
100
0
.
I .
. . . I 2 , . . --.
.-.
I
200
300
400
500
Binding Energy(uncorrected)/eV
Widascanphotoelectron spectrumat grazing emission for an air-cleavedmica surface after exposure to 600 langmuirs of ammonia vapor at rmm temperature. All the characteristic peake of principal constituents of mica can be seen with a very large C 1s and a small N 1s peak. Figure 5.
corrected for charging shift with respect to the K 2~312 l e ~ e l .Similar ~ ~ ~ ~results were obtained for ammonia adsorption on the thoroughly degassed, vacuum-cleaved mica surface. With the appearance of the N le peak, n o shifts in the XPS peaks of the main constituents of mica were noticed. The N 1s peak vanished after the mica sample was heated to a temperature of 470K, in agreement with the TPD data. Very few XPS measurements have been reported on adsorption of ammonia on solid acids. Defosse and Canessonz7observed that pyridine adsorption on NH4-Y zeolites produced two N 1s peaks in the ranges of 397.5398.3 eV and 399.5-400.5 eV, assigned respectively to adsorption at Lewis acid and BrGnsted acid sites. Rogers et aLZ8found a N 1s peak at 400.3 eV for ammonia adsorption on an oxidized aluminum surface. It was suggested that the peak was due to molecularly held ammonia. Borade et al.% from their work on pyridine adsorption on ZSM-5 zeolites, found three N la peaks at 398.7,400.0,and 401.8eV, which were assignedrespectively to Lewis acid sites, relatively weak acid sites, and strong Bronsted acid sites. It is thus not possibleto draw definite conclusion about the nature of sites on mica responsible for the N 1s peak, but the very broad shape of the peak points to a distribution of acidic sites over a range of strengths.
Conclusion In this work, the inert mica surface has been shown to be active toward ammonia, suggestingexistence of a small (26) Kantachewa, M.; Albano, E. V.; Ertl, G.; Knozinger, H. Appl. Catal. 1983,8, 71. (27) Defoam, C.; Canemon, P. J. Chem. SOC.,Faraday %M. 1 1976, 72, 2666. (28) Rogem, J. W., Jr.; Campbell, C. T.; Hance, R. L.; Whita, J. M. Surf. Sci. 1980, 97, 425. (29) Borade, R.; Sayari, A.; Adnot, A.; Kaliaguhe, S. J. Phys. Chem. 1990,94,6989.
Adsorption of Ammonia on Mica Surfaces
number of acidic sites on the surface. A possible mechanism has been proposed in line with the mechanism of ammonia adsorption on zeolites. However it should be recognized that application of Redhead's formulation to analyzethe TPD results has many weaknesses. A rigorous analysis requires varying the heating rates at least by 2 or 3 orders of magnitude, which was not possible in this work due to experimental limitations. Much more work will be
Langmuir, Vol. 8, No. 9, 1992 2289
needed for an accurate characterization of the acidic sites on mica and for explaining the interesting TPD results.
Acknowledgment. The experimentalpart of this work was done at the laboratory of Dr. David 0. Hayward of Imperial College of Science, Technology and Medicine, London. The help and encouragement from Dr. Hayward are gratefully acknowledged.
